Simplified equalizer for twisted pair channel

ABSTRACT

A 100Base-TX detection system is presented which takes advantage of the form of the frequency response of the channel to provide a simplified filter for producing an output signal with reduced distortion. Utilizing the nature of the frequency response function of category-5 twisted pair cabling, a finite impulse response linear equalizer or an infinite impulse response decision feedback equalizer having as few as two multipliers is implemented.

BACKGROUND

1. Field of the Invention

This invention relates to simplifying the equalizer needed to combat theintersymbol interference present in a digital communication system.

2. Background

The dramatic increase in desktop computing power driven byintranet-based operations and the increased demand for time-sensitivedelivery between users has spurred development of high speed EthernetLANs. 100BASE-TX Ethernet, using category-5 copper wire, and the newlydeveloping 1000BASE-T Ethernet for Gigabit/s transfer of data overexisting category-5 copper wire require new techniques in high speedsymbol processing. Gigabit per second transfer can be accomplishedutilizing four twisted pairs and a 125 megasymbol/s transfer rate oneach pair where each symbol represents two bits. Twisted pair coppercables are also used in wide-area networking (WAN) and digitalsubscriber loop data communication applications. With ever increasingneed for bandwidth, technologies that support high data transfer ratesacross twisted pair cables are gaining wide acceptance. 100Base-TX (fastEthernet), 1000Base-T transmission over long haul copper (also known asGigabit Ethernet) and digital subscriber loop technologies all transmitdata at high transmission rates over twisted copper pairs.

Physically, data is transferred using a set of voltages where eachvoltage represents one or more bits of data. Each voltage in the set ofvoltages is referred to as a symbol and the whole set of voltages isreferred to as a symbol alphabet.

One system of transferring data at high rates is Non Return to Zero(NRZ) signaling. In NRZ signaling, the symbol alphabet {A} is {-1, +1}.A logical "1" is transmitted as a positive voltage while a logical "0"is transmitted as a negative voltage. At 125 M symbols/s, the pulsewidth of each symbol (i.e. the positive or negative voltage) is 8 ns.

Another system for high speed symbol data transfer is referred to asMLT3 signaling and involves a three voltage level system. (See AmericanNational Standard Information system, Fibre Distributed Data Interface(FDDI)--Part: Token Ring Twisted Pair Physical Layer Medium Dependent(TP-PMD), ANSI X3.263:199X). The symbol alphabet for MLT3 is {A}={-1, 0,+1}, corresponding to the set of voltages {-V, 0, V}. The voltage V istypically 1 V.

In MLT3 transmission, a logical "1" is transmitted by either a -1 or a+1 symbol while a logic "0" is transmitted as a 0 symbol. A transmissionof two consecutive logic "1"s does not require the system to passthrough zero in the transition. A transmission of the logical sequence("1", "0", "1") would result in transmission of the symbols (+1, 0, -1)or (-1, 0, +1) depending on the symbols transmitted prior to thissequence. If the symbol transmitted immediately prior to the sequencewas a +1, then the symbols (+1, 0, -1) are transmitted. If the symboltransmitted before this sequence was a -1, then the symbols (-1, 0, +1)are transmitted. If the symbol transmitted immediately before thissequence was a 0, then the first symbol of the sequence transmitted willbe a +1 if the previous logical "1" was transmitted as a -1 and will bea -1 if the previous logical "1" was transmitted as a +1.

In the ideal MLT3 system, the transmit driver simply sends a voltagepulse corresponding to the symbol being transmitted. The pulse is ofduration 8 nanoseconds for each one of the transmit symbols and has afinite rise/fall time of three to five nanoseconds (See AmericanNational Standard Information system, Fibre Distributed Data Interface(FDDI)--Part: Token Ring Twisted Pair Physical Layer Medium Dependent(TP-PMD), ANSI X3.263:199X).

The detection system in the MLT3 standard, however, needs to distinguishbetween three voltage levels, instead of two voltage levels in a twolevel system. The signal to noise ratio required to achieve a particularbit error rate is higher for MLT3 signaling than for two level systems.The advantage of the MLT3 system is that the power spectrum of theemitted radiation from the MLT3 system is concentrated at lowerfrequencies and therefore more easily meets FCC radiation emissionstandards for transmission over twisted pair cables. Other communicationsystems may use a symbol alphabet having more than two voltage levels inthe physical layer in order to transmit multiple bits of data using eachindividual symbol.

A block diagram of a typical digital communication transmission systemis illustrated in FIG. 1. In FIG. 1, the transmitted data is representedby the symbol sequence {a_(k) }. The transmitted symbols in the sequence{a_(k) } are members of the symbol alphabet {A}. In the case of threelevel MLT3 signaling, the symbol alphabet {A} is given by {-1, 0, +1}.The index k represents the time index for that symbol, i.e., at sampletime k, the symbol being transmitted is given by a_(k). The channelresponse is represented by the channel transfer function f(z). Thechannel function f(z) is the Z-transformation of the sampled timeresponse of the channel.

In FIG. 1, the transmitted symbols {a_(k) } enter the channel 1. Thesignal output from the channel 1, x_(k), is a linear distortion of thetransmitted symbols {a_(k) }, the distortion being described by thechannel transfer function f(z). The signal x_(k) is summed in adder 2with a noise sample n_(k) to form the signal y_(k). The noise samples{n_(k) } represent the random noise on the transmission line. The signaly_(k), suffering from both the channel distortion and the random noise,is then input to the detector 3. Detector 3 inputs the distorted signalsy_(k), counteracts the effects described by the channel transferfunction f(z), and outputs a sequence of detected symbols {a_(k) }.

FIG. 2 shows a typical 100Base-Tx transmitter. The transmit data path ina 100Base-TX transceiver (IEEE 802.3u Standard) consists of a physicalcoding sub-layer (PCS) 11, and a physical medium dependent (PMD)sub-layer 12. The PCS 11 contains a medium independent interface (MII) 4and a 4B5B (rate 4/5) encoder 5. The medium independent interface 4 isthe interface between the transceiver and the media access controller(MAC). The 4B5B encoder 5 guarantees sufficient transitions in thetransmit data for the purpose of robust clock recovery in the receiverand generates Ethernet control characters. The data rate at the outputterminal of the PCS 11 is 125 Mhz due to the rate penalty associatedwith the 4B5B encoder 5. The physical medium dependent portion 12 of the100Base-TX transmit data path consists of a scrambler 6, binary to MLT3converter 7, and a transmit driver 8 which outputs a 1V peak-to-peaksignal onto the twisted pair 10 through an isolation transformer 9. Thetransmit symbol sequence {a_(k) } is generated in the binary to MLT3converter 7.

It is assumed that the channel model represented by f(z) includes theeffect of transmit and receive filtering. In addition, the transmissionchannel is assumed to be linear in that two overlapping signals simplyadd as a linear superposition. Therefore, the channel transfer functionpolynomial can be defined as

    f(Z)=f.sub.0 +f.sub.1 Z.sup.-1 +f.sub.2 Z.sup.-2 + . . . +f.sub.N Z.sup.-N,(1)

where f₀, . . . , f_(j), . . . , f_(N) are the polynomial coefficients.The polynomial coefficient f_(j) represents the dispersed component ofthe (k-j)th symbol present in the kth received sample and N is a cut-offinteger such that f_(j) for j>N is negligible. The polynomial f(Z)represents the Z-transformation of the sampled frequency response of thetransmission channel. In Equation 1, Z⁻¹ is considered to be a one clockperiod delay. See A. V. OPPENHEIM & R. W. SCHAFER, DISCRETE-TIME SIGNALPROCESSING 1989.

The noiseless output of the channel at sample time k is then given by

    x.sub.k =f.sub.0 *a.sub.k +f.sub.1 *a.sub.k-1 + . . . f.sub.N *a.sub.k-N,(2)

where, without loss of generality, f₀ can be assumed to be 1. Thus, thechannel output signal at time k depends not only on transmitted data attime k, but past values of the transmitted data. This effect is known as"intersymbol interference" (ISI). See E. A. LEE AND D. G. MESSERSCHMITT,DIGITAL COMMUNICATIONS (1988).

Intersymbol interference is a result of the dispersive nature of thecommunication channel. The IEEE LAN standards require that systems becapable of transmitting and receiving data through at least 100 metersof category-5 cable. FIG. 3A shows a transmission symbol stream with theeffects of dispersion. FIG. 3B shows the power spectrum of the dispersedpulse versus frequency. In a 100 meter cable, the signal strength at theNyquist frequency of 62.5 Mhz is reduced nearly 20 db at the receivingend of the cable. Given this dispersion, a single transmitted symbol mayaffect several received symbols at the output of the wire.

The noise element of the signal is represented by the sequence {n_(k) }.Therefore, the noisy output signal of the channel is given by

    y.sub.k =x.sub.k +n.sub.k,                                 (3)

where the noise samples {n_(k) } are assumed to be independent andidentically distributed Gaussian random variables (see LEE &MESSERSCHMITT) with variance equal to σ².

Most state-of-the art communication systems use two types of detectorsfor combating the ISI described by equation (2). These two detectors,Linear Equalization and Decision Feedback Equalization, are shown inFIG. 4A.

A finite impulse response linear equalizer having m+1 multipliers isillustrated in FIG. 4B. In FIG. 4B, the symbol y_(k) is inputted to adelay array 10 having delays (D₁ through D_(m)) which, at each stage,delay the symbol by one time period. A set of multipliers 20 havingmultipliers M₀ through M_(m) multiply each of the m+1 symbols in thearray of delays D₁ through D_(m) by a corresponding coefficient C₀through C_(m). The adder 30 adds together the output signals frommultipliers M₀ -M_(m) to obtain the resulting signal

    a.sub.k =C.sub.0 y.sub.k +C.sub.1 y.sub.k-1 + . . . +C.sub.m y.sub.k-m.(4)

The signal a_(k) ', from the linear equalizer is inputted to slicer 40which decides on the output symbol a_(k). The output symbol a_(k) is thesymbol from the symbol alphabet {A} which best approximates the inputsignal a_(k) '.

The multiplier coefficients, C₀ through C_(m), define a transferfunction T given by

    T=C.sub.0 +C.sub.1 Z.sup.-1 + . . . +C.sub.m Z.sup.-m.     (5)

The coefficients C₀ through C_(m) may be chosen by an intelligentalgorithm in an adaptive implementation in order to optimize thefunctioning of the equalizer. A zero-forcing linear equalizer (ZFLE) hasa transfer function T given by the inverse of the frequency response ofthe channel. A minimum mean squared error based linear equalizer(MMSE-LE) optimizes the mean squared error between the transmitted dataand the detected data, and hence finds a compromise between theun-canceled ISI of the output signal of the equalizer and the outputnoise variance.

FIG. 4C illustrates a typical finite impulse response Decision FeedbackEqualizer (DFE) with N_(ff) multipliers in the feed-forward filter andN_(fb) multipliers in the feed-back filter. The input signal y_(k) isinputted to the feed-forward filter 100. The resulting signal from thefeed-forward filter is added with the negative of the resulting signalfrom the feed-back filter 200 in adder 300. The added signal a_(k) isinputted to slicer 400 which determines the output symbol a_(k) of theequalizer.

In feed-forward filter 100, the input signal y_(k) is inputted to afeed-forward delay array having delays D₁ ^(ff) through D_(Nff-1) ^(ff).Each delay delays the signal by one period so that the delay array 101stores N_(ff) -1 past input signals. Each of the stored signals ismultiplied by a corresponding coefficient C₀ through C_(Nff-1) bymultipliers M₀ ^(ff) through M_(Nff-1) ^(ff). The output signals fromthe multipliers M₀ ^(ff) through M_(Nff-1) ^(ff) are added together inadder 103 so that the signal inputted to adder 300 on line 301 is givenby

    a.sub.k "=C.sub.0 y.sub.k +C.sub.1 y.sub.k-1 + . . . +C.sub.Nff-1 y.sub.k-Nff+1.                                            (6)

The feed-back filter 200 inputs the output symbol a_(k) to a feed-backdelay array 201 having delays D₀ ^(fb) through D_(Nfb-1) ^(fb). Thefeed-back delay array 201 stores N_(fb) past determined symbols,a_(k-Nfb) through a_(k-1). The output symbols of the feed back delayarray 201 are inputted to multipliers 202, M₀ ^(fb) through M_(Nfb-1)^(fb) respectively. The resulting signals from multipliers 202 are addedin adder 203 so that the input signal of adder 300 on line 302 is givenby

    a.sub.k '"=b.sub.0 a.sub.k-1 +b.sub.1 a.sub.k-2 +b.sub.Nfb-1 a.sub.k-Nfb.(7)

Adder 300 adds the input signal on line 301 with the negative of theinput signal on line 302 to obtain a_(k) '=a_(k) "-a_(k) '", which isreceived by slicer 400. Slicer 400 decides on the output symbol a_(k).The output symbol a_(k) arrived at by slicer 400 is the symbol in symbolalphabet {A} which most closely approximates the signal a_(k) ' at theinput terminal of slicer 400.

The DFE operates on the principle that if the past transmitted data iscorrectly detected, then the ISI effect of these past data symbols canbe canceled from the current received signal prior to detection. For azero-forcing DFE, the feed-forward transfer function is set to 1 (i.e.,C₀ =1 and C₁ through C_(m) are 0 in the finite impulse response filterof FIG. 4C), and the feedback transfer function is given by [f(z)-1],f(z) being the channel transfer function. Practical implementation ofdecision feed-back equalizers utilize finite impulse response (FIR)feed-back filters. A finite impulse response filter implements atransfer function which is finite in duration. Infinite impulse response(IIR) filters, those that implement a transfer function which isinfinite in duration, have difficulty implementing algorithms foradaptively adjusting the multiplier coefficients.

Since past detected data samples contain no noise, DFE does not sufferfrom noise enhancement while the linear equalizer does. However, DFEsuffers from error propagation; i.e., if one of the past detectedsymbols is incorrect, then the effects of that error propagate to moresymbol decisions in the future.

Also, because the equalizer is a feedback equalizer, pipelining of thefeed-back filtering operation is not possible, unlike a linear equalizerwhose operation can be pipelined. In particular, a linear equalizerdepends only on input signals and therefore can use several clock cyclesto perform the computational functions necessary to arrive at an outputsignal. The effect of using several clock cycles is to enable high speedimplementation of the equalizer by splitting the computational load ofthe equalizer over several clock cycles. A decision feedback equalizer,however, depends on the output of previous symbols to determine thecurrent symbol, i.e., a_(k-1) is necessary to determine a_(k).Therefore, all computations to determine the symbol a_(k) need to beaccomplished within a single clock cycle, preventing pipelining of theequalizer.

Mathematically, the frequency response of the twisted pair cable can bemodeled as e⁻β. The exponent β is αl, (jf)^(1/2) where α is the cablecoefficient, l is the length of the cable in meters, and f is thefrequency in Mhz. For a category-5 twisted pair cable, α isapproximately 3.7×10⁻³ /(m√MHz) The overall frequency response of thesystem, including the channel, the TX shaping and the transformer, isgiven by

    H(f)=H.sub.T (f)e.sup.-β,                             (8)

where H_(T) (f) includes the effects of transmit shaping and transformerfrequency response. These effects include the effect of an analog todigital converter, a low pass filter, and a high pass filter. H_(T) (f)can be approximately modeled by ##EQU1## where T=1/125 MHz, f_(L) is ofthe order of 25-50 Khz, and f_(H) is approximately 85 Mhz for the fastEthernet transmission system.

A sampled impulse response of the channel (a folded spectrum) is givenby

    H.sub.S,τ (f)=(1/T)Σ.sub.k H.sub.T (f+k/T)e.sup.-j2πfτ(10)

where -0.5/T≦f<0.5/T and τ is the timing phase of the sampler that isselected by the clock recovery circuitry in the receiver. See LEE &MESSERSCHMITT.

A typical equalizer implements the channel function f(z) calculated bysetting f(z=e^(j2)πfT)=H_(S),τ (f). This process results in the designof an equalizer having 12 or more multipliers.

SUMMARY OF THE INVENTION

In accordance with the invention, an equalizer which takes advantage ofthe characteristics of the frequency response of the channel ispresented. Applicant has observed that the frequency response of thechannel is approximated by a function having a series of poles in thedenominator. The number of multipliers required to implement theequalizer is equal to the number of terms in the series of poles and,therefore, is minimal.

In the preferred embodiment, a linear equalizer using only twomultipliers is presented. In a second embodiment, a decision feedbackequalizer utilizing only two multipliers is presented. Both equalizersexploit the observed channel function having a series of poles in thedenominator.

A detector embodying this invention has an equalizer with an inputterminal to receive an input signal suffering from channel distortion.The channel distortion is described by a channel function with adenominator polynomial of order L and having K denominator polynomialcoefficients, L being a positive integer greater than 1 and K being apositive integer less than or equal to L. The equalizer implements achannel function with L delays and K multipliers, each of the Kmultipliers having a multiplier coefficient equal to a corresponding oneof the K denominator polynomial coefficients. The equalizer outputs acorrected signal in response to the K denominator polynomialcoefficients and the input signal.

A finite impulse response (FIR) linear equalizer implementing thedenominator polynomial is the preferred embodiment of the invention. Aninfinite impulse response (IIR) decision feedback equalizer implementingan IIR filter in the feed-back section is presented as anotherembodiment of the invention.

The invention and its embodiments are further described with the figuresand the accompanying discussion.

DESCRIPTION OF THE FIGURES

FIG. 1 shows a typical digital communication system.

FIG. 2 shows a 100Base-TX transceiver data path for transmitting data.

FIG. 3A shows dispersion across the transmission channel.

FIG. 3B shows the magnitude response of a typical 100 meter category-5cable.

FIG. 4A shows two typical detectors used in a digital communicationsystem.

FIG. 4B shows a typical linear equalizer.

FIG. 4C shows a typical decision feedback equalizer.

FIG. 5 shows a 100Base-TX receiver according to the present invention.

FIG. 6 shows the preferred detection system utilizing a linearequalizer.

FIG. 7 shows a decision feedback equalizer according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, a linear equalizer is presentedwhich requires a minimal number of multipliers in the multiplier array.The multiplier coefficients are advantageously chosen to reduce thenumber of multipliers required.

For category-5 cabling used in fast Ethernet transmission, it isempirically observed that the frequency response of the channeldescribed by Equation 10 can be approximated as

    H.sub.S,τ (z)=gz.sup.-M /(1+b.sub.1 z.sup.-1 +b.sub.2 z.sup.-2 + . . . +b.sub.L z.sup.-L)                                        (11)

where z=e^(j2)πfT, g is the channel flat loss factor, M is a fixed delayin baud periods, {b_(i) } are the coefficients of a denominatorpolynomial and L is a positive integer greater than 1. The denominatorpolynomial of order L, 1+b₁ z⁻¹ +b₂ z⁻² + . . . +b_(L) z^(-L), displayedin Equation 11 is an expansion in a series of poles with the coefficientb_(i) multiplying the ith order term z^(-i). The denominator polynomialcoefficients, {b_(i) }, depend on the overall sampled spectrum.

                  TABLE 1                                                         ______________________________________                                        Coefficients as a function of cable length                                    Cable Length                                                                             flat-loss   As-Calc.   As-Meas.                                    (meters)   g           b.sub.1, b.sub.2, b.sub.3                                                                b.sub.1, b.sub.2, b.sub.3                   ______________________________________                                         0         0.9771      -0.0614,   -0.0078,                                                           +0.0090,   +0.0078,                                                           +0.0090    +0.0078                                     20         0.7676      -0.1002,   -0.1016,                                                           -0.0157,   -0.0078,                                                           -0.0056    -0.0078                                     40         0.5743      -0.2375,   -0.2344,                                                           -0.0253,   -0.0156,                                                           -0.0195    -0.0234                                     60         0.4360      -0.3593,   -0.3281,                                                           -0.0189,   -0.0156,                                                           -0.0325    -0.0313                                     80         0.3248      -0.4912,   -0.4531,                                                           +0.0116,   +0.0078,                                                           -0.0457    -0.04696                                    100        0.2409      -0.6323,   -0.5625,                                                           +0.0698,   +0.0234,                                                           -0.606     -0.0703                                     ______________________________________                                    

The parameters in Equation 11 are given in Table 1. Table 1 gives valuesfor the flat loss factor g, the first three denominator polynomialcoefficients {b1, b2, b3} as-calculated using Equation 10, and valuesfor the first three denominator polynomial coefficients {b1, b2, b3} asempirically measured for varying lengths of category-5 cable. Thediscrepancy between the as-calculated and as-measured values for thedenominator polynomial coefficients is attributable to the model notperfectly representing the channel. However, the model is sufficient toprovide the basis on which to implement a detection system.

The Ethernet receiver is designed to "undo" the effects of the frequencydistortion H_(S),τ (f). From Equation 11, a linear equalizer implementedwith the transfer function

    E=(1+b.sub.1 z.sup.-1 +b.sub.2 z.sup.-2 + . . . +b.sub.L z.sup.-L)(12)

compensates for the distortion of the channel. The effect of the flatloss g, shown in Table 1 for various cable lengths, is countered byautomatic gain control circuitry in the receiver.

In the preferred embodiment of the implementation of the invention, L=3is found to be a good compromise between performance and complexity.Therefore, the denominator polynomial has terms only through z⁻³ and canbe implemented using only three delays and requiring up to threemultipliers. In addition, the preferred equalizer is implemented as afinite impulse response linear equalizer and is therefore amenable topipelining for use in VLSI architectures. In the preferredimplementation, the linear equalizer is implemented using pipelining.

It is further found from the as-measured denominator polynomialcoefficients shown in Table 1 that the following simplifications in thecoefficients result in negligible loss of performance:

    b.sub.1 ≦0 for all cable lengths;

    b.sub.2 =0 for all cable lengths; and

    |b.sub.3 |≦1/4for all cable lengths.

Therefore, the preferred equalizer, the linear equalizer, implements thetransfer function

    E=(1+b.sub.1 z.sup.-1 +b.sub.3 z.sup.-3),                  (13)

which is implemented using only two multipliers. In general, adenominator polynomial having L terms will require K multipliers toimplement where K is a positive integer greater than 1 but less than orequal to L. An alternative embodiment of the invention is a decisionfeedback equalizer having a feed-forward filter implementing thetransfer function 0.1 and a feed-back filter implementing the transferfunction [H_(S),τ (z)-1].

The linear equalizer output signal at sample time k is given by

    a.sub.k =y.sub.k +b.sub.1.sup.k y.sub.k-1 +b.sub.3.sup.k y.sub.k-3(14)

where y_(k) is the equalizer input signal at sample time k. In thepreferred embodiment, the coefficients b₁ ^(k) and b₃ ^(k) are adjustedadaptively for each sample time k. The output signal from a slicer, thedecoded MLT3 decision based on the equalizer output signal, at sampletime k is given by ##EQU2##

In the preferred implementation, the coefficients b₁ ^(k) and b₃ ^(k)are adaptively chosen by a least mean squares (LMS) algorithm. Measuredcoefficients for various cable lengths are given in Table 1 but areadjusted at each sample time to optimize the linear equalizer. Thecoefficients depend on the cable length, transmit shape 8 andtransformer 9 characteristics (see FIG. 2). In the linear equalizerembodiment, coefficients are updated for sample time (k+1) according tothe following recursion:

    b.sub.1.sup.k+1 =b.sub.1.sup.k -γ(a.sub.k '-a.sub.k)y.sub.k-1

    b.sub.3.sup.k+1 =b.sub.3.sup.k -γ(a.sub.k '-a.sub.k)y.sub.k-3(16)

where the constant γ is the update constant. The update constant γcontrols the rate of correction of the multiplier coefficients, which,as seen from Equation 16, is based on the calculated error, a_(k)'-a_(k), in the equalizer output. The update recursion shown in Equation16 allows the receiver to react to the changes in the channel bycorrecting for the error. The channel changes with various factorsincluding age and environmental temperature variations.

Several considerations determine the value of the update constant γ. Ifγ is too low, then the update recursion will be too slow in convergingon optimum values for the multiplier coefficients b₁ ^(k+1) and b₃^(k+1). If γ is too large there will be a larger error in the multipliercoefficients b₁ ^(k+1) and b₃ ^(k+1) with respect to their optimumvalues. The continuous feedback in the update recursion, which iscontrolled by γ, causes the multiplier coefficients b₁ ^(k+1) and b₃^(k+1) to oscillate around optimum values with a variation dependent onthe value of γ. In the preferred embodiment, γ is chosen to be large,1×10⁻³, on start-up of the receiver and is reduced to about 1×10⁻⁴ forcontinuous operation of the receiver. In this way, rapid convergence tooptimum values of the multiplier coefficients is achieved and thereceiver responds to variations in the channel while oscillations aroundthe optimum values of the multiplier coefficients are minimized.

FIG. 5 shows a 100 Base TX receiver utilizing this invention. The inputsignal from the twisted copper pair is input to an amplifier 400 whichcompensates for the channel flat loss factor g by amplifying the inputsignal by a gain of 1/g. The gain is adjusted by gain control 407 inorder to optimize the receiver function. Measured relative values of gfor several cable lengths are given in Table 1.

The anti-aliasing filter 401 prevents anti-aliasing by passing the inputsignal through a low pass filter to reject out-of-band noise. The analogto digital converter (ADC) 402 samples and holds the input signal for aduration of 8 ns. The digitized signals y_(k) are then input toequalizer 403. In equalizer 403, the effects of the channel distortionare countered and the equalizer 403 outputs signal a_(k) '. If equalizer403 is the linear equalizer implementation then Equation 14 isimplemented approximately and if the equalizer 403 is a decisionfeedback equalizer then the feed-forward filter implements 1 and thefeed-back filter implements approximately [H_(S),τ (z)-1]. In addition,if a decision feedback equalizer is implemented line 408 is inserted toprovide the feed-back section of the equalizer 403 with the result fromslicer 404. Slicer 404 inputs signal a_(k) ' from equalizer 403 anddecides on the output symbol a_(k) by implementing Equation 15.

The multiplier coefficients {b_(i) } are adaptively chosen in thecoefficient update block 405. The multiplier coefficients correspond tothe denominator polynomial coefficients shown as a function of cablelength in Table 1. Coefficient update 405 implements Equation 16 for alinear equalizer and adjusts the multiplier coefficients on each timeperiod. Clock recovery 406 tracks the timing of the circuit and adjuststhe timing phase τ for the sample and hold function of the analog todigital converter (ADC) 402. Clock recovery 406 adjusts the timing phaseτ by estimating the zero crossings in the signal a_(k) '. Gain control407 adjusts the gain of multiplier 400 by comparing the modulus ofsignal a_(k) ' with a target threshold value. The gain of multiplier 400compensates for the channel flat loss factor g in Equation 11.

FIG. 6 shows the preferred implementation of the equalizer 403 in the100Base TX receiver of FIG. 5. The equalizer implements the transferfunction of Equation 13. Equalizer 410 in FIG. 6 is a finite impulseresponse linear equalizer having two multipliers 414 and 415 and threedelays 411, 412 and 413, each of which delays the signal by one clockperiod. The signal y_(k) is input to delay 411 and to adder 416. Theoutput signal of delay 411, y_(k-1), is input to delay 412 andmultiplied by b₁ ^(k) in multiplier 414. The output signal of multiplier414, b₁ ^(k) y_(k-1), is input to adder 416. The output signal of delay412, y_(k-2), is input to delay 413. The output signal of delay 413,y_(k-3), is multiplied by b₃ ^(k) in multiplier 415. The output signalof multiplier 415, b₃ ^(k) y_(k-3), is inputted to adder 416. The outputsignal of adder 416, y_(k) +b₁ ^(k) y_(k-1) +b₃ ^(k) y_(k-3), is thesignal a_(k) ' of Equation 14 which is input to slicer 404. In general,an implementation of the linear equalizer also includes animplementation of a second transfer function in addition to the abovedescribed transfer function. In the prefered implementation, the secondtransfer function is 1.

FIG. 7 shows an infinite impulse response decision feedback equalizer420 according to this invention. The decision feedback equalizer 420 isalso implemented with only two multipliers, multipliers 426 and 427, inthe implementation of the denominator polynomial minus 1 as part of thefeed-back filter 429 of the decision feedback equalizer 420. In FIG. 7,the feed-forward portion of the decision feedback equalizer 420 has beenset to one so that a_(k) " is equal to the input signal y_(k) and isinput directly to adder 421. In general, the feed-forward portion of thefeedback equalizer implements a feed-foward transfer function.

The output signal from slicer 404, a_(k), of FIG. 5 is input, throughline 408, to adder 422. Adder 422 adds the output symbol from the slicer404, a_(k), to the output signal of feed-back filter 429, a_(k) '". Theoutput signal of adder 422 is input to delay 423. The output signal ofdelay 423 is input to delay 424 and multiplier 426. Multiplier 426multiplies the output signal from delay 423 by b₁ ^(k) and inputs theresulting signal to adder 428. The output signal from delay 424 is inputto delay 425. The output signal from delay 425 is input to multiplier427. Multiplier 427 multiplies the output signal of delay 425 by b₃^(k). The output signal from multiplier 427 is added to the outputsignal from multiplier 426 in adder 428. Adder 421 subtracts the outputsignal from adder 428, a_(k) '", from the input symbol y_(k) to obtaina_(k) ' which is input to slicer 404.

Delays 423, 424 and 425, multipliers 426 and 427 and adder 428 offeed-back filter 429 implements the transfer function b₁ ^(k) z⁻¹ +b₃^(k) z⁻³. The feedback provided by line 430 causes the feed-back filter429 to implement the channel function

    T(z)=(b.sub.1 z.sup.-1 +b.sub.3 z.sup.-3)/(1+b.sub.1 z.sup.-1 +b.sub.3 z.sup.-3).                                                (17)

The channel function in Equation 17 is the negative of the channelresponse of Equation 11--without the fixed delay or flat loss factor andwith L=3 and b₂ =0--minus 1. The output signal of the equalizer, a_(k)', therefore, is

    a.sub.k '=y.sub.k +b.sub.1.sup.k (a.sub.k-1 -a.sub.k-1 '"+)+b.sub.3.sup.k (a.sub.k-3 -a.sub.k-3 '")                                 (18)

where the coefficients b₁ ^(k) and b₃ ^(k) are adaptively adjusted bycoefficient updater 405 to optimize the equalizer, a_(k) " is the outputsignal of the feed-forward filter, y_(k), and a_(k) '" is the outputsignal of the feed-back filter 429. As before, slicer 404 implementsEquation 15 and decides on the output symbol a_(k).

An alternative embodiment of the decision feedback equalizer implementsa channel function corresponding to f(z)-1, -T(z) of Equation 17. Inthat case, a_(k) '=y_(k) +b₁ ^(k) (a_(k-1) +a_(k-1) '"+)+b₃ ^(k)(a_(k-3) +a_(k-3) '"). In both embodiments, adder 421 adds the inputsignals y_(k) and a_(k) '" together. In this alternative embodiment,adder 422 also adds both of the input signals, a_(k) and a_(k) '",together as opposed to subtracting a_(k) '" from a_(k) as shown in FIG.7.

The coefficients b₁ ^(k) and b₃ ^(k) in the decision feedback equalizerare adaptively chosen by coefficient updater 405 according to thefollowing recursive equation:

    b.sub.1.sup.k+1 =b.sub.1.sup.k -γe.sub.k (a.sub.k-1 ·-a.sub.k-1 '")

    b.sub.3.sup.k+1 =b.sub.3.sup.k -γe.sub.k (a.sub.k-3 ·-a.sub.k-3 '")                                  (19)

where e_(k) =a_(k) '-a_(k). The update constant γ is again chosen, withthe same considerations as in Equation 16 for the linear equalizer, tooptimize convergence to the optimum multiplier coefficients for theequalizer. Note that in the implementation of Equation 19 (see FIG. 5)the coefficient updater 405 inputs the output signal of the feed-backfilter of decision feedback equalizer 420, a_(k) '". Also, in a moregeneral IIR decision feedback equalizer, if the feed-back multiplierspresent in the feed-back filter of the decision feedback equalizerimplement the transfer function f₁ z⁻¹ +f₂ z⁻² + . . . +f_(j) z^(-j) + .. . +f_(L) z³¹ L, then the coefficient f_(j) ^(k+1) is adaptively chosenusing the recursive equation f_(j) ^(k+1) =f_(j) ^(k) -γe_(k) (a_(k-j)·-a_(k-j) '").

The embodiments of the invention described above are demonstrative only.Modifications of these embodiments obvious to one skilled in the art arewithin the scope of this application. As such, the scope of thisapplication is limited only by the following claims.

I claim:
 1. A detector system comprising an equalizer, the equalizercomprising:an input terminal for receiving an input signal, the inputsignal suffering from a channel distortion described by a channelfunction having a denominator polynomial of order L hazing K denominatorpolynomial coefficients, L being a positive integer greater than 1 and Kbeing a positive integer greater than or equal to 1 and less than orequal to L; an implementation of a transfer function, the transferfunction being substantially the inverse of the channel function, theimplementation having L delays coupled in series and K multipliers, eachof the K multipliers having a multiplier coefficient corresponding toone of the K denominator polynomial coefficients, each of the Kmultipliers coupled to receive an output signal from one of the Ldelays, and an adder coupled to receive output signals from each of theK multipliers; and an output terminal coupled to the implementation foroutputting a corrected signal in response to the K denominatorpolynomial coefficients and the input signal, wherein:L=; and the Ldelays include a first delay, a second delay, and a third delay coupledin series, each of the first delay, the second delay and the third delaydelaying the input signal by one clock, cycle, the first delay receivingthe input signal; wherein: K=2; the K multipliers include a firstmultiplier and a second multiplier, the first multiplier having a firstcoefficient and the second multiplier having a second coefficient, thefirst multiplier receiving a first delayed signal from the first delayand outputting at a first output terminal the first delayed signalmultiplied by the first coefficient, the second multiplier receiving athird delayed signal from the third delay and outputting at a thirdoutput terminal the third delayed signal multiplied by the secondcoefficient; and the adder is coupled to the input terminal of theequalizer, the first output terminal and the third output terminal andwhich outputs the sum of the first delayed signal multiplied by thefirst coefficient, the third delayed signal multiplied by the secondcoefficient, and the input signal as the corrected signal of theequalizer.
 2. The system of claim 1, further comprising:a slicer havingan input terminal coupled to the output terminal of the equalizer toreceive the corrected signal and having an output terminal foroutputting an output symbol in response to the corrected signal.
 3. Thesystem of claim 2, wherein the equalizer has a coefficient inputterminal for receiving the corresponding multiplier coefficient of eachof the K multipliers, and further comprising:a coefficient updaterhaving a coefficient output terminal coupled to the coefficient inputterminal of the equalizer, the coefficient updater adaptively adjustingthe corresponding multiplier coefficient of each of the K multipliers inresponse to the input signal, the corrected signal and the outputsymbol.
 4. The system of claim 1, wherein the first coefficient and thesecond coefficient are adaptively chosen by a coefficient updatercoupled to the equalizer.
 5. The system of claim 1, wherein the firstcoefficient is less than or equal to 0 and the second coefficient has amagnitude less than or equal to about 0.25.
 6. The system of claim 1,further including an implementation of a second transfer function, anoutput terminal of the implementation of the second transfer functionbeing coupled to an input terminal of the implementation of the transferfunction.
 7. A detector system comprising an equalizer, the equalizercomprising:an input terminal for receiving an input signal, the inputsignal suffering from a channel distortion described by a channelfunction having a denominator polynomial of order L having K denominatorpolynomial coefficients, L being a positive integer greater than 1 and Kbeing a positive integer greater than or equal to 1 and less than orequal to L; an implementation of a transfer function, the transferfunction being substantially the inverse of the channel function, theimplementation having L delays coupled in series and K multipliers, eachof the K multipliers having a multiplier coefficient corresponding toone of the K denominator polynomial coefficients, each of the Kmultipliers coupled to receive an output signal from one of the Ldelays, and an adder coupled to receive output signals from each of theK multipliers; an output terminal coupled to the implementation foroutputting a corrected signal in response to the K denominatorpolynomial coefficients and the input signal; a slicer having an inputterminal coupled to the output terminal of the equalizer to receive thecorrected signal and having an output terminal for outputting an outputsymbol in response to the corrected signal; wherein the equalizerincludes a decision feedback equalizer having feed-forward portion and afeed-back portion, the feed-back portion having a feed-back inputterminal for receiving the output symbol, the feedback portion being theimplementation of the transfer function; and wherein L=3; and the Ldelays include a first delay, a second delay, and a third delay coupledin series, each of the first delay, the second delay and the third delaydelaying the output symbol by one clock cycle, the first delay having aninput terminal coupled to the feed-back input terminal; wherein K=2; theK multipliers include a first multiplier and a second multiplier, thefirst multiplier having a first coefficient and the second multiplierhaving a second coefficient, the first multiplier receiving a firstdelayed signal from the first delay and outputting the first delayedsignal multiplied by the first coefficient, the second multiplierreceiving a third delayed signal from the third delay and outputting thethird delayed signal multiplied by the second coefficient; a first addercoupled to output terminals of the first multiplier and the secondmultiplier and having an output terminal for outputting an outputsignal, the output signal being the sum of the first delayed signalmultiplied by the first coefficient and the third delayed signalmultiplied by the second coefficient; a third adder coupled to theoutput terminal of the first adder and receiving the output symbol, thethird adder outputting the difference between the output symbol and theoutput signal of the first adder to the input terminal of the firstdelay; and a second adder coupled to the output of the first adder andthe feed-forward portion of the equalizer, the second adder outputtingthe corrected signal, the corrected signal being the sum of an outputsignal from the feed-forward portion of the equalizer and the outputsignal of said first adder.
 8. The system of claim 7, wherein the firstcoefficient and the second coefficient are adaptively adjusted by acoefficient updater coupled to the equalizer.
 9. The system of claim 8,wherein the first coefficient is less than or equal to 0 and the thirdcoefficient has a magnitude less than or equal to about 0.25.
 10. Thesystem of claim 7, wherein the feed-forward portion of the equalizerimplements a transfer function equal to
 1. 11. The system of claim 9,wherein the feed-foward portion of the equalizer implements a transferfunction equal to 1.